The scanning probe microscope (SPM) is a general term that includes scanning tunneling microscopes and atomic force microscopes. The underlying principles of SPMs are to make use of tunneling currents, chemical forces, and electrostatic forces between the microscope probes and the test sample. When the probe scans a surface of a sample, the change in the interaction between the probe and the sample can reflect a great deal of information on the sample, such as surface morphology, electronic density distribution, and surface potential.
Over the last 30-plus years of development, types of scanning probe microscopes have continually expanded and improved and have become indispensable research tools in fields such as surface science, materials science, physics, chemistry, and biology. Over the course of their development, various types of scanning probe microscopes have generally specified placement of the SPMs in different kind of environments such as ultra-high vacuum, low temperature, magnetic fields, electric fields, microwave, and optical fields. Through adjusting and regulating environmental parameters, the performance of scanning probe microscopes can be improved and achieve more precise measurements.
As for a scanning probe microscope under a high magnetic field, many physical processes and phenomena can occur to a test sample under the action of an external magnetic field such as electron spins or even nuclear spins that start to precess or flip. Under the current technologies, the magnetic field is achieved by using a superconducting coil magnet made of superconducting material with zero resistance effect. However, the superconducting magnet needs to be maintained under a low-temperature environment, since the superconducting phase transition temperatures are relatively low (usually less than 10 K). In the field of scanning probe microscope, because the probe and the sample are very close to each other, may be as low as a scale of a single atom size, the SPMs are extremely sensitive to external vibrations and noise from the environment.
Thus, for scanning probe microscopes under a high magnetic field environment, superconducting magnets are usually placed in a liquid helium Dewar cryostat. The cooling is achieved through the latent heat during phase change of liquid helium at 4.2 K, in order to achieve the superconducting phase transition temperature of superconducting magnet, and then large electrical current is introduced to generate high magnetic field. Because the liquid helium itself does not generate vibrations, it can be directly integrated with the scanning probe microscope to form a low-temperature, high magnetic field environment. However, liquid helium is a globally scarce resource that is very expensive. Because liquid helium is a non-renewable resource, in recent years, the price of liquid helium has been rising steadily. In light of this, cryogen-free closed-cycle cooling systems that do not consume liquid helium, such as Gifford-McMahon and pulse tube cryogen-free cooling systems, are being commonly used worldwide for the cooling of superconducting magnets. However, because of the relatively strong mechanical vibrations and noise of existing cryogen-free closed-cycle cooling systems, the scanning probe microscopes, which are extremely sensitive to vibrations and noise, cannot utilize those superconducting magnets cooled by existing cryogen-free closed-cycle cooling systems. So, it is desirable to develop scanning probe microscopes in high magnetic environments that can utilize cryogen-free closed-cycle cooling systems.